Acoustic resonators are components which were subject of numerous technical studies. Conventionally, one distinguishes between Surface Acoustic Resonator (SAW) and Bulk Acoustic Resonator (BAW). In SAWs, the acoustic resonator is located on the surface of a semiconductor product while, in BAWs, it lays inside a volume delimited between a lower electrode and a higher electrode so that the acoustic wave develops in this volume.
Because of their efficiency, acoustic resonators are frequently used in radio frequency (RF) filtering and in particular in mobile telephony. They are however likely to serve in many other applications but the clear obstacle to their general use reside in the difficulty to be integrated with other electronics circuitry in a same semiconductor product.
Indeed, the components which are produced in the manufacturing lines show a great dispersion in their characteristics, and this even for the BAW type resonator which show to be more suitable for a direct integration on a silicium substrate.
Conventionally, one solves the problem of the dispersion in the characteristics of the components by selectively sorting the manufactured components in order to keep the particular one which fully complies with specifications.
One may also try to solve the problem of dispersion of the characteristics by integrating those resonators within a tunable structure. This permits on one hand, to compensate for the shortfalls of the manufacturing process and, on the other hand, to take advantage of new functions which result from the tuning possibilities given to the resonator.
U.S. Patent Application Publication No. 2004/0033794 entitled “RESONATOR CONFIGURATION”, published on Feb. 19, 2004 discloses a process for tuning a resonator located on an integrated circuit by means of a second resonator of reference which is also located on the same substrate. The reference resonator is used in a VCO type circuit (Voltage Control Oscillator) for the purpose of generating local oscillation.
U.S. patent application Ser. No. 11/025,599 entitled “INTEGRABLE ACOUSTIC RESONATOR AND METHOD FOR INTEGRATING SUCH RESONATOR”, filed on Dec. 29, 2004, discloses a new acoustic component which is easy to integrate on a silicium substrate. Indeed, one uses an acoustic resonator which is based on a dielectric mirror located on a silicium substrate. To this end, an acoustic resonator based on a dielectric medium arranged on a reflecting element, such as a Bragg mirror or a receiver for example, is used. Layers having different acoustic properties and different dielectric constants are stacked on a silicon substrate. Such an acoustic element is known as a Surface Mounted Resonator (SMR).
Alternatively, the resonator could be of the Film Bulk Acoustic Resonator type (FBAR), namely a resonator located above a cavity to allow the reflection of acoustic waves and to avoid damping thereof.
FIGS. 1A, 1B and 1C respectively illustrate the electrical model of a BAW type resonator (showing two L-C type resonating circuits, respectively series and parallel, comprising elements Lm, Cm and C0 with resistors Raccess, Rm and Ro), the characteristic curve of the impedance as well as the formulas giving the computation of the series resonance frequency and parallel resonance frequency of that component. As this can be seen, the impedance curve shows a resonance frequency Fs (series resonance frequency) and an anti-resonance frequency Fp (or parallel resonance frequency) which are very close to each other.
Generally speaking, the difference between the anti-resonance frequency and the resonance frequency in a filtering circuit defines the pass band of that circuit and the tuning of the filter based on such resonators shows to be difficult to achieve. One solves this problem by using a Tunable Resonator Component (TRC) described in the above mentioned U.S. patent document. As this is seen in FIG. 2A, one connects in parallel an inductor to the resonator. This inductor has a first technical effect of shifting to the right the useful anti-resonance frequency so as to move it away from the series resonance frequency. This achieves a filtering circuit which can be tuned in frequency. The inductor moreover results in the generation of a second anti-resonance frequency, which is located at the left of the series resonance frequency as this can be seen in FIG. 2B. This second anti-resonance frequency is a parasitic effect which one tries to keep outside of the useful band, particularly by locating the resonance frequency at the middle of the range of frequencies defined by the two anti-resonance frequencies.
In order to complete the arrangement of the Tunable Resonator Component (TRC) known in the art, one introduces in series a variable capacitive element, such as a varactor. This varactor allows the tuning of the series resonance frequency and, finally, the adjustment of the frequency of the TRC.
That circuit operates in a satisfactory manner but with some drawbacks.
Firstly, that circuit requires, for every BAW resonator, one inductor—e.g., an integrated spiral inductor - which is known to occupy a non negligible surface on the semi-conductor substrate. When a complex band pass filter has to be carried out, based on a set of components illustrated in FIG. 2A, it becomes necessary to allocate an expensive surface of silicium.
Secondly, integrated inductors on silicium substrate have a low quality factor resulting in additional losses and jeopardizing the performance of the BAW resonators. Inductors with higher quality factors do exists but they required a different substrate, such as a glass substrate, and this is a clear obstacle for the purpose of achieving a low costs manufacturing process.